The speed at which the sphere of a nuclear blast expands depends on various factors, including the size of the explosion, the type of nuclear weapon used, and the environment in which it occurs. When a nuclear explosion occurs, there are different phases of the blast wave that propagate outward from the point of detonation.
The initial phase is known as the "fireball" or the "prompt radiation phase." During this phase, an intense release of energy in the form of light, heat, and radiation occurs. The fireball expands rapidly, initially at supersonic speeds. The exact velocity of the fireball can vary depending on the yield of the nuclear weapon, but it can reach speeds of several thousand meters per second (or several kilometers per second) in the early moments after the detonation.
As the fireball expands and cools down, it transitions into the next phase known as the "blast wave." The blast wave is a rapidly expanding shockwave of high-pressure air and debris. The speed of the blast wave depends on factors such as the yield of the nuclear weapon, the altitude of the detonation, and atmospheric conditions. In general, the blast wave can travel at several hundred meters per second (or several kilometers per hour).
It's important to note that the speeds mentioned above are approximate and can vary depending on the specific circumstances of the nuclear explosion. Additionally, the destructive effects of a nuclear blast extend far beyond the initial blast wave, including thermal radiation, ionizing radiation, and other long-term effects.
Nuclear explosions are highly complex phenomena, and accurate modeling and prediction of their effects require specialized knowledge and tools. The specific details of nuclear blast dynamics and the associated speeds can vary depending on the characteristics of the explosion, making it important to consult reliable scientific and military sources for precise information in a given context.